This invention relates to semiconductor devices in general and more particularly, to a pressure sensitive piezo-optical switch fabricated from a porous semiconductive material.
It has long been known that the application of a pressure or stress can affect not only the energy gap of semiconductors but also the population of the various equi energy valleys, thereby altering the quantity of energetic carriers. The sensitivity to stress exhibited by semiconductor materials has enabled the fabrication of a wide variety of devices including semiconductor diode microphones, transistor microphones, stress sensing tunnel diodes, and a whole class of piezoresistive transducers. See, for example, U.S. Pat. No. 4,204,185 entitled INTEGRAL TRANSDUCER ASSEMBLIES EMPLOYING THIN HOMOGENEOUS DIAPHRAGMS, issued to Anthony D, Kurtz and Richard A. Weber on May 20, 1980 and assigned to the assignee herein for an illustrative transducer device employing the piezoresistive effect.
Reference is made to an article by Serge Haroche and Jean-Michel Raimond entitled "Cavity Quantum Electrodynamics" and appearing in the Scientific American, April 1993. In that article, the authors describe a condition in which an excited atom is confined in a cavity so small that the atom is unable to radiate light because the wavelength of the oscillating field it would "like" to produce cannot fit within the boundaries. As long as the atom can not emit a photon, it remains at the same energy level. When so confined, the atom-cavity system apparently oscillates between two states, one consisting of an excited atom and no photon and the other of a de-excited atom and a photon trapped in the cavity.
Reference is also made to an article appearing in Vol. 256 of the Materials Research Society Symposium Proc. (1992), J. S. Foresi and T. D. Moustakas in which the phenomenon of quantum confinement in microcrystalline silicon and its effect on piezoresistance is examined. The authors of that article note that although the microstructure of microcrystalline silicon is similar to that of granular metals, the effect of strain on the conductivity of microcrystalline silicon is contrary to that which has been observed in such metals. Specifically, while the conductivity of granular metals increases with compressive strain as the metal grains are brought closer together and the tunneling distance through the insulating barriers surrounding the metal grains decreases, conductivity decreases of up to 100 percent were observed in microcrystalline silicon films under compression, suggesting that such films might be utilized for highly sensitive strain gauge applications. Foresi et al. attributed the decreases in conductivity to quantum confinement of the energetic carriers in the small silicon crystallites and to the change of the ground state energy in the quantum wells with strain.
It has also been observed that in certain instances, porous silicon exhibits unique properties which are superior to those of bulk silicon. For example, it has been found by light transmission measurements that the band gap in microporous silicon is increased by 0.5 eV over that of bulk silicon. One explanation which has been advanced for the increase in band gap energy is that the energetic carriers are confined in quantum sized wells defined by the microporous structure.
The semiconductive properties of SiC have also been the subject of recent attention. Its wide band-gap, high thermal conductivity, high breakdown electric field, and high melting point make SiC an excellent material for high temperature and high power applications. In U.S. patent application Ser. No. 07/957,519now U.S. Pat. No. 5,298,767 entitled Porous Silicon Carbide (SiC) Semiconductor Device issued on Mar. 29, 1994, the present applicants disclose methods of forming a new semiconductor material, porous SiC by electrochemical anodization of monocrystalline SiC. It is expected that the microporous structure of porous silicon carbide exhibits the same capacity for quantum confinement as that of porous silicon.
It is therefore an object of the present invention to advantageously utilize porous semiconductor materials, such as silicon and silicon carbide, in the fabrication of improved piezo-optical pressure sensitive switch devices.